Molecular characterization and increased expression of the Nile tilapia, Oreochromis niloticus (L.), T-cell receptor beta chain in response to Streptococcus agalactiae infection

The complete cDNA sequence of the Nile tilapia T‐cell receptor (TCR) β chain was cloned using 5′ RACE. The full‐length, 1263‐bp cDNA contained a 942‐bp open reading frame (ORF) encoding a 314‐amino‐acid protein. Sequence analyses revealed that the Nile tilapia TCR β chain contains four conserved cysteine residues involved in the formation of disulphide bridges and a conserved amino acid motif believed to be important for assembly and signalling of the TCR αβ/CD3 complex, both of which are normally found in the TCR β chain of other vertebrates. As detected using semi‐quantitative and quantitative RT‐PCR, the highest expression level of TCR β was detected in the thymus. Interestingly, Streptococcus agalactiae significantly induced the up‐regulation of the TCR β chain, and the strongest up‐regulation was detected in the brain and peripheral blood leucocytes (PBLs). In in vitro experiments, concanavalin A and Aeromonas hydrophila were found to significantly increase the expression of the TCR β chain in PBLs after 48 h (P